Remove errors for invalid escape sequences in tagged template literals

[uhyo]

Happy hacktoberfest! 🎃🥳

Fixes #39715.
Also gives a partial fix to #396.

Example

Code

String.raw`\x \012`;

`\x \012`;

Current Behavior

src/backslashx.ts:1:14 - error TS1125: Hexadecimal digit expected.

1 String.raw`\x \012`;
               

src/backslashx.ts:3:4 - error TS1125: Hexadecimal digit expected.

3 `\x \012`;
     


Found 2 errors.

New Behavior

src/backslashx.ts:3:4 - error TS1125: Hexadecimal digit expected.

3 `\x \012`;
     

src/backslashx.ts:3:7 - error TS1392: Octal escape sequences are not allowed in template strings.

3 `\x \012`;
        


Found 2 errors.

(Error disappeared from tagged template literal, and new error for octal escape sequence in untagged template literal)

Implementation Detail

With this PR, invalid escape sequences do not emit errors when they are first scanned. Untagged template literals with invalid escape sequence in it are re-scanned to emit syntax errors.

[uhyo]

Happy new financial year (in Japan)!! 🌸🥳
A silent remainder on this. I rebased on the latest master.

[elibarzilay]

I don't think that this is a good approach since it complicates the template parsing code (and only when tags are used?) but it can be simpler to handle all strings in one place instead since they all go through the same place. I posted a comment listing how this can be done.

(Oh, I'm guessing that the tag thing is to deal with raw-tagged strings (#42887); in any case, I think that it's better to fix that case after dealing with the general string case.)

[uhyo]

Thank you for your reviews!
I cannot afford to deal with the new approach though, closing this PR.